Recently, exploding growth of mobile data communication traffic due to proliferation of smart phones and other smart devices has accelerated mobile network operators to deploy or consider deploying various technologies to increase network capacity. One of key enabling technologies is MIMO (Multi-Inputs and Multi-Outputs) technology.
Specifically, MIMO technology has already been standardized in Third Generation Partnership Project (3GPP) Long-Term Evolution (LTE) and LTE-Advanced standards. MIMO technology comprises two sub-categories, namely, Single-User-MIMO (SU-MIMO) and Multi-User MIMO (MU-MIMO). SU-MIMO technology enables transmission of multiple layers of data on the same time and frequency resource between a base station and a single user terminal. On the other hand, MU-MIMO technology, while offering the same benefit as SU-MIMO, can additionally enable data transmission on the same time and frequency resource between a base station and multiple user terminals. Therefore, in order to maximize network capacity, mobile network operators worldwide are considering deployment of MU-MIMO technology.
MU-MIMO is the most effective when channels from a base station to all user terminals can be precisely known. For a downlink communication direction (from base station to user terminals), which dominates most of mobile data communication traffic, this can be realized easily especially in Time-Division Duplexing (TDD) system comprising a base station configured to exploit channel reciprocity property between channels in the downlink direction and channels in an reverse (uplink) direction.
FIG. 1 illustrates a typical example of a system in a related art which channel reciprocity between one base station and one user terminal is exploited by the base station.
Specifically, referring to FIG. 1, the user terminal 20R transmits Uplink Reference Signal (Uplink RS, which is equivalent to Sounding Reference Signal (SRS) in LTE system) from M antennas 21R (operation S11).
The base station 10R receives from the user terminal 20R Uplink RS transmitted from each of the user terminal's antennas 21R (operation S12).
Then, the base station 10R estimates channel from every user terminal's antenna 21R to every base station's antenna 11R to obtain Uplink channel matrix HU (operation S13). HU is an NT×M impulse response matrix, where NT is the number of antennas 11R of the base station 10R and M is the number of antennas 21R of the user terminal 20R.
Finally, the base station 10R invokes channel reciprocity property to obtain Downlink channel matrix HD by using the Uplink channel matrix HU,HD=HHU,where HD is an M×NT impulse response matrix and the superscript H denotes complex conjugate transpose or Hermitian transpose (operation S14).
Once the base station 10R obtains all Downlink channel matrices to all user terminals, the base station 10R can create a precoding matrix (hereinafter also referred to as “Base-station-created precoding matrix”) for each user terminal. Data of one user terminal with the precoding matrix applied thereto is made not to interfere with data of other user terminals. There are several methods to create the precoding matrix.
FIG. 2 is a diagram for illustrating an example of Block-Diagonalization (BD) precoding method described in Non-patent Literature (NPL) 1. FIG. 2 illustrates a case where the base station 10R creates two Base-station-created precoding matrices F1 and F2 for User terminals 1 and 2 (12-1 and 12-2), respectively, when respective Downlink channel matrices H1 and H2 to User terminals 1 and 2 are known at the base station 10R.
To ensure that data of one user terminal do not interfere with those of the other user terminal, the base station 10R creates the Base-station-created precoding matrix (F1, F2) for the user terminal that is equivalent to a null-space matrix of the other user terminal's channel (V2(n), V1(n). That is,F1=V2(n), and F2=V1(n),where Vi(n) (i=1,2) forms orthogonal basis for null space of Hi (channel matrix from base station to user terminal i).
More specifically, assuming that the number of antennas 11R of the base station 10R is NT, the number of antennas (21R-i) (i=1, 2) of i-th user terminal (20R-i) is Nri (=M), Nr=Nr1+Nr2(=2M), and Hi is a Downlink channel matrix (Nri×NT impulse response matrix with a rank Li), Hi is decomposed using SVD (singular value decomposition),Hi=UiDi[Vi(s)Vi(n)]H,where Ui (i=1, 2) is an (Nr−Nri)×(Nr−Nri) Unitrary matrix,
Di (i=1, 2) is an (Nr−Nri)×NT matrix having Li (=rank of Hi) positive singular values and zeros in diagonal elements and having zeros in off-diagonal elements,
Vi(s) (i=1, 2) is an NT×Li matrix having, as column vectors, NT-orthonormal vectors corresponding to Li positive singular values, and
Vi(n) (i=1,2) is an NT×(NT−Li) matrix having, as column vectors, NT-orthonormal vectors corresponding to zero singular values.
In the base station 10R, each modulated codeword (codeword from a coder not shown is modulated by a modulator not shown) is mapped onto one or more layers. The number of layers is less than or equal to the number of transmit antenna ports. Each layer is mapped by precoding-matrix onto one or more transmit antenna ports associated with physical transmission antennas. Each of adders 13-1 to 13-NT connected to each of NT antennas adds associated mapped layers for user terminals 1 and 2.
When one user terminal (20R1/20R2) receives signals transmitted from the base station, the one user terminal does not experience interference from the other user terminal.
Therefore, the user terminal can adjust a receiving matrix G to only extract multiple layers of data intended for the user terminal itself, by for example, using a Zero-Forcing (ZF) or Minimum Mean-Squared Error (MMSE) criterion.
Note: The following gives an outline about a Zero-Forcing (ZF) receiver. Assuming that the received signal (vector) r observed at the user terminal is modeled as:y=Hx+v, where x is a transmit vector, H is a channel matrix from the base station to the user terminal, and v is a noise vector (additive white Gaussian noise (AWGN)). When channel state information is perfect, the ZF estimate of the transmitted vector can be expressed asy−=G(Hx+v)=x+Gv, where G=[HH H]−1 HH is the ZF receiver. The superscript −1 denotes inverse of a matrix. H+=[HH H]−1 HH is a pseudo inverse matrix (left inverse of H, that is, H+H=I, where I is an Identity Matrix).
Although interference between user terminals (Inter-user interference: IUI) can be handled by the Base-station-created precoding matrices, the base station still needs to select appropriate Modulation and Coding Scheme (MCS) for each data layer in order to maximize each user terminal's throughput, and thus network capacity.
The selection of MCS by the base station is for example performed as follows.
The base station first acquires received channel quality observed by each user terminal conditioned on the Base-station-created precoding matrix. The received channel quality reflects a power of a desired signal with respect to a power of interference and noise experienced at the user terminal. The interference in this case refers to an undesired signal that may be generated due to imperfect nulling of Inter-user interference by the base station that is serving the user terminal or transmission of signals from neighboring base stations that are serving different user terminals.
Then, the received channel quality is used to determine the highest MCS that satisfies a predefined data transmission error rate. The received channel quality metric most commonly used is Signal to Interference pluses Noise Ratio (SINR), defined as the desired signal power divided by the total power of power of interference and noise.
FIG. 3 depicts an example of MCS-SINR mapping table that depicts an example of how SINR is mapped to MCS. FIG. 3 is taken from NPL2's Section 7.2.3 Channel Quality Indicator (CQI) definition.
Referring to FIG. 3, in “Modulation”, there are provided QPSK (Quadrature Phase Shift Keying) and 16QAM (Quadrature Amplitude Modulation) and 64QAM.
“Code rate” is k/n, for every k bits of useful information, while the coder generates totally n bits of data, of which n-k are redundant.
“Spectral efficiency (usage) (C) (bit/s/Hz)” is a net bit-rate (bit/s) (useful information rate excluding error-correcting codes) or maximum throughput, divided by a bandwidth in Hertz of a communication channel.
Regarding “SINR”, from the well known Shannon's channel capacity equation, Spectral efficiency (C) is given as C=log2(1+SINR). Accordingly, SINR(dB) is given as log10(2C−1).
Therefore, in conclusion, the base station needs to obtain SINR observed by the user terminal conditioned on the Base-station-created precoding matrix in order to select an appropriate MCS for each data layer and maximize channel (network) capacity.
The mobile communication system, such as LTE, has some mechanism in which a user terminal reports received SINR observed by the user terminal to the base station. NPL2's Section 7.2 UE procedure for reporting Channel State Information (CSI) describes such mechanism in details.
FIG. 4 is a simplified sequence chart illustrating the CSI reporting operations disclosed in NPL2. Here, the system according to the example includes one base station and two user terminals only for the sake of simplicity.
Both user terminals 1 and 2 (20R-1 and 20R-2) first receive Downlink Reference Signal (Downlink RS, which is equivalent to Channel State Information Reference Signal (CSI-RS) in LTE system) broadcasted from each of the base station's antennas to every user terminals (operation S21).
Then, the user terminals 1 and 2 (20R-1 and 20R-2) estimate, respectively Down link channel matrices H1 and H2 from the base station's antennas to the user terminals 1 and 2 (20R-1 and 20R-2), based on the received Downlink RS, respectively (operations S22-1 and 2).
The base station 10R transmits a request for reporting channel quality information to the respective user terminals 1 and 2 (20R-1 and 20R-2) (operations S23-1 and 2).
After that, the user terminals 1 and 2 (20R-1 and 20R-2) create respectively precoding matrices Fuser1-created and Fuser2-created (hereinafter also termed as User-created precoding matrix) by using the estimated channel in order to maximize received SINR (operations S24-1 and 2).
Next, the user terminals 1 and 2 (20R-1 and 20R-2) estimate received SINR for each data layer conditioned on the User-created precoding matrices Fuser1-created and Fuser2-created, respectively (operations S25-1 and 2).
Finally, the user terminal 1 (20R-1) reports both the User-created precoding matrix Fuser1-created, and the received SINR conditioned on Fuser1-created to the base station (operation S26-1) and the user terminal 2 (20R-2) reports both the User-created precoding matrix Fuser2-created and the received SINR conditioned on the User-created precoding matrix Fuser2-created to the base station (operation S26-2).
More specifically in LTE, the user terminal performs the process of creating the User-created precoding matrix by selecting one precoding matrix from a predefined set of precoding matrices (candidates: called codebook) known to both the user terminal and the base station. The user terminal reports the User-created precoding matrix and the corresponding SINR to the base station using Precoding Matrix Indicator (PMI) index and Channel Quality Indicator (CQI) index, respectively. Higher the CQI index (from 0 to 15) reported by the user terminal to the base station, the base station uses higher modulation scheme (from QPSK to 64QAM) and higher code rate to achieve higher efficiency.
In Patent Literature (PTL) 1, there is disclosed a MIMO system including eNodeB and K user equipments, wherein the eNodeB carries out precoding and realizes a space division multiplex among K user equipments. In PTL1, the eNodeB comprises an apparatus including uplink channel estimation means for receiving a signal from one or more user apparatuses in a space division multiplex group to estimate an uplink channel characteristic based on the received signal, calibration information determination means for determining calibration information between the uplink channel characteristic and downlink channel characteristic, and precoding means for determining a downlink precoding matrix based on the uplink channel characteristic and calibration information, and transmitting a downlink signal to the one or more user apparatuses in a space division multiplex group.
In PTL2, there is disclosed a MIMO system in which many base stations each can receive channel state information (Channel Status Information: CSI) from a mobile station, and can estimate a channel using the channel state information. Each base station is adapted to generate antenna weights separately based on such a channel estimation result, and perform precoding to transmit precoded signal.
In PTL3, there is disclosed a method including a step for feed backing static characteristic of CSI and CSI error to a base station (BS) from a user equipment (UE), a step for generating a multi-user precoding matrix and a scheduling scheme according to the static characteristic of the feed-backed CSI and CSI error, and a step for carrying out multi-user precoding and scheduling to user data by using the generated multi-user precoding matrix and scheduling scheme.    [PTL 1]    Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. JP2012-531132A    [PTL 2]    Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. JP2011-509040A    [PTL 3]    Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. JP2010-537598A